WASHINGTON, DC - JULY 19: Sen. Al Franken (D-MN) attends the Democratic Policy and Communications Committee hearing in the Capitol building on July 19, 2017 in Washington, DC. The hearing dealt with the subject of &quot;Democracy for Sale&quot; and how they feel that the campaign finance system allows foreign governments to buy influence in the U.S. Elections and what can be done about it. (Photo by Joe Raedle/Getty Images)

Joe Raedle

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Twelve Democratic senators -- including Ohio Sen. Sherrod Brown -- called on Sen. Al Franken to resign Wednesday, marking the first time any senator has called on the Minnesota Democrat to leave office as allegations of sexual harassment against him continue to mount.

Franken is expected to make an announcement on Thursday, multiple outlets reported.

In a statement on Facebook, New York Sen. Kirsten Gillibrand wrote: "While Senator Franken is entitled to have the Ethics Committee conclude its review, I believe it would be better for our country if he sent a clear message that any kind of mistreatment of women in our society isn't acceptable by stepping aside to let someone else serve."

Sens. Claire McCaskill of Missouri, Heidi Heitkamp of North Dakota Patty Murray of Washington, Maggie Hassan of New Hampshire, Kamala Harris of California, Tammy Baldwin of Wisconsin, Debbie Stabenow of Michigan and Mazie Hirono of Hawaii also joined in the call for Franken to resign. He has been accused of inappropriately touching several women.

Sen. Bob Casey of Pennsylvania was the first male Democratic senator to call on Franken to resign just after noon Wednesday. Sen. Joe Donnelly of Indiana and Brown called for Franken to step down later in the day.

"I am grateful to the victims who have had the bravery to come forward," Brown said. "Their courage has created a movement that is bringing about change. I have listened to them. I have listened to my female colleagues, to women I work with and women in my life. And I agree the time has come for Sen. Franken to step aside."

Murray's backing is significant because she's a member of Democratic leadership and is the highest ranking woman in the Senate.

The Senate Ethics Committee is investigating Franken following an account described by Leeann Tweeden, a morning news anchor on KABC radio in Los Angeles, which described Franken groping and forcibly kissing her during a USO tour in 2006, before Franken became a senator. After that initial account, several other women came forward to say Franken inappropriately touched them. Franken has repeatedly apologized about behavior that he said "crossed a line" for some women. The second-term senator has also said that he has taken thousands of photos with people over the years and that while he doesn't remember specific pictures or campaign events, any inappropriate behavior was unintentional.

At least six women -- three named and three unnamed -- have accused Franken of inappropriately touching them. The most recent accusation came in a Politico report Wednesday, in which, a woman who chose not to be identified alleged Franken tried to forcibly kiss her after a taping of his radio show in 2006. Franken released a statement categorically denying the accusation. "This allegation is categorically not true and the idea that I would claim this as my right as an entertainer is preposterous," the Minnesota senator said. "I look forward to fully cooperating with the ongoing ethics committee investigation." CNN has not verified the accusations in the Politico report.​

The calls for Franken to resign come one day after Democratic Rep. John Conyers of Michigan announced he would retire immediately. Conyers had also faced multiple allegations of sexual harassment by former employees, accusations Conyers vehemently he denied.